1. Field of the Invention
The present invention generally relates to a method and an apparatus for fast finding and optimizing the best resolution of an optical scanning device.
2. Background Description
Generally, an optical scanning system employs many components such as an image capturing device, lenses and so on, to assemble a scanning module with high precision. The scanning module can generate and record an image of an object after proper alignment and calibration. The scanning module further would be able to transform the captured image to digital signals with carrying out the visual image. In this case, the scanning process is completed. Therefore, a precise alignment and good assembly quality are a very important cause to the scanning module.
In FIG. 1, there is shown a traditional alignment and assembly method of a scanning module. The scanning module (not shown in FIG. 1) includes at least a scanning module 10 having a document glass 101, a lens 102 and an image capturing device 103. The image capturing device 103 generally is a Charged-Coupled Device (CCD). In addition, there are additional supporting device needed to assemble a traditional optical scanning system, such as calibration device 11, an adjusting device 12 connected to the image capturing device 103. The adjusting device 12 references signals from the image capturing device 103 for displaying parameters or signals, in order to inform the assembling technicians to identify the status of assembly.
Consequently, FIG. 2 shows the schematic demonstration of the calibration device 11. There is a print 110 on the surface of the calibration device 11. The print shows multi parallel lines, for calibration and detection purpose.
Further, please refer to FIG. 3. FIG. 3 shows a char of Modulation Transfer Function (MTF)—Position of the Lens. The MTF is a known mathematic fiction. If the value of MTF 31 is higher, the resolution of the scanning module is greater. On the other hand, lower value of the MTF means lower resolution of the scanning module. The horizontal dimension of the chart represents the position of the lens 32. Usually, the curve 30 of the chart of the MTF-PL will be different for different scanning module 10. When the researching and developing engineers determine which scanning module is implemented, the curve 30 of the chart is fixed. Therefore, for a fixed shape of the chart of MTF-PL, there is a maximum value for the MTF. In theory, as shown in the figure, the best resolution of the scanning module will be achieved if the lens 34 is placed on the point where the value of MTF is the maximum. In practice, it is very difficult to assemble the scanning module precisely having the best resolution, namely, making the position of lens in the point where the MTF achieves the maximum value, due to the misalignment. Thus, if the value of the MITF can be set above a certain level, the resolution of the scanning module is acceptable by the user. In FIG. 3, if the point where the lens is placed is in the tolerant range 36, the value of the MTF of the scanning module will be kept above the MTF tolerant value 35. In view of the above, the position of the lens is the major issue of the resolution of an optical scanning device.
Accordingly, the assembly of the traditional optical scanning device includes the steps of the followings. Firstly, the assembling technician mounts the document glass 101 and the image capturing device 103. The lens 102 is able to move along a specified direction linearly, as shown in FIG. 1. The lens is further temporarily fixed in a predetermined position. A calibration device 11 is placed on the document glass 101. The calibration device 11 contains a print 110. Please refer to FIG. 2. The optical scanning device further includes a light source (not shown in the figure). The light source provides a light illuminated on the document glass 101 and later reflected by the calibration device 11. The light is transmitted via an optical route 104 to the lens 102, and generated an image on the image capturing device 103. The image capturing device 103 generates digital signals. In this point, the adjusting device 12 calculates the MTF by referencing the digital signals and displays the result. In this case, the value of MTF presents the resolution of the calibration device 11, which is placed in the document glass 101. The assembling technician is therefore able to adjust the position of the lens in order to get the maximum value of MTF, and fixes the lens at the position where the value of MTF is maximum or above the MTF tolerant value. If the value of MTF is maximum, the best resolution of the optical scanning device is achieved.
The calibration device 110, described in the above for adjusting the scanning system, only has one print, and the print contains plural parallel lines. The vector on X-axis and the vector on Y-axis of the lines give the adjusting device 12 basis to calculate the value of MTF for representing the resolution of the X-axis and Y-axis. Moreover, some calibration devices only provide plural parallel lines in one direction, such as X-axis or Y-axis. In this case, the assembling technician only get the reference resolution in one direction. Practically, due to the misalignment of the assembly, the axis of the lens 102 is usually unable to be perpendicular to the document glass. Thus, the axis of the lens has an inclined angular with the X-axis or Y-axis, as shown in FIG. 4A. Upmost, there are possibly containing two inclinations along X-axis and Y-axis. In this case, the X-axis resolution and the Y-axis resolution of the same point are different. Therefore, the best position of the lens cannot be obtained in this respect. Since a standard resolution can be obtained via traditional skills, the quality of assembled scanning devices would be reduced if the position of the lens only relies on one direction, such as X-axis or Y-axis.
In view of the above, it is important to provide a method and an apparatus for optimizing the best resolution of an optical scanning device in this industry.